Abstract
Subglacial processes of the last Scandinavian Ice Sheet in Fennoscandia were investigated on two different scales: a detailed case study in southwestern Finland, and a generalized study in the central parts of the ice sheet. Previously reconstructed time-transgressive glacial flow patterns were correlated with different types of glacial flow-parallel features and lithostratigraphic units. The comparison of orientations of each flow-parallel feature with relative ages of different flow patterns enabled estimations of relative distances of the orientation-generating processes in relation to the receding ice margin. This method was also used to infer sedimentary environments of each lithostratigraphic facies identified. Generalized lithostratigraphic sequences were compiled in Fennoscandia using sections described previously. In areas of ice streams a sandy till showing mature deformation structures forms till sheets, drumlins and some ribbed moraines. The facies is overlain by occasional glaciofluvial deposits and partly washed deformation till both indicating abundant excess water at the ice-bed interface. In interlobate areas, glaciofluvial material is under- and overlain by a silty till showing ductile deformation structures. In places, uppermost thin clayey till, interpreted to be a subglacial melt-out till, mantles underlying till and subglacial glaciofluvial deposits. The characteristics of different units are controlled by ice-flow velocities, subglacial hydrology and deformation type of the substratum. The time-transgressive origin of till beds and intervening washed sediment, as well as divergent ice-flow patterns, is emphasized. Ice flow was concentrated in streams which generated marginal fan-shaped lobes. Interstream areas of low basal velocities and melting rates were left between adjacent ice streams. In areas of ice streams, fast basal ice-flow velocities continuously generated heat and meltwater which flowed at first in the sediment and closer to the margin also in thin discontinuous sheets at the ice-bed interface. Excess water may have subsequently formed a linked cavity system and channelized to a tunnel network initiating esker deposition or incision of tunnel channels. Most processes operated continuously during deglaciation, with spatial and temporal differences in their activity. The mode of subglacial drainage changed periodically during deglaciation, and was probably controlled by the volume of subglacial meltwater available. In some periods, sheet flow of basal water deposited major marginal complexes, and subsequently tunnel networks were re-established. Esker pattern and sedimentation rates of proglacial varved clays indicate that tunnel drainage of subglacial water was partly episodic.
Published Version
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